1. Field of the Invention
The invention relates to a method for manufacturing a resonant cavity for laser.
2. Description of the Related Art
The conventional ion laser includes two reflecting optical elements placed at either end of a laser tube. In general, the optical elements are coupled directly to the laser tube; that is, the mirrors are mounted to the ends of the tube, with a glass frit, or solder glass joining the reflective portion of the element directly to the end of the laser tube. During the glass frit process, temperatures can reach up to 450.degree. C. U.S. Pat. No. 4,893,314, inventors Shull, et al., assigned to Spectra-Physics, Inc., shows a mounting alternative wherein the laser optics are placed on an internal mount. A mirror seat is inserted into the interior of the laser tube and the optical element is mounted on the mirror seat with the reflective portion of the element on the opposite side of the mounting area.
Optical elements for a laser resonator may be manufactured by coating a substrate, made of a material such as glass, with a series of dielectric films to develop the desired reflectance/transmittance of the mirror, depending on whether the mirror is to be used as a high reflectance mirror or as an output coupler. As is well known, such coatings are generally comprised of a plurality of layers of dielectric material alternating between materials with high and low indices of refraction having optical thicknesses of .lambda./2 or .lambda./4, thereby defining a reflective surface based on the classic Fabrey-Perot interferometer. The number, index of refraction, and optical thickness of the layers is determined by the desired reflectivity or transmittance of the optical element.
Lasers utilizing resonator optics mounted directly to the laser tube are generally incompatible for use with dielectric coatings having high indices of refraction. This is due to the fact that the coated side of the substrate must be coupled directly to the tube. Exposure of high index material to the 450.degree. C. frit temperature renders the coating useless. Coatings with high index dielectrics are preferable for making broad band or "white light" visible laser optics, and some ultra-violet laser applications because of the ease with which broadband reflectors may be manufactured. In a broad band or "white light" laser, the optical elements are required to reflect and transmit energy over a relatively large bandwidth encompassing red, green and blue regions of the visible spectrum. In a conventional laser tube having tube mounted laser optics, a large number of dielectric layers are required to achieve the desired reflective/transmissive characteristics over a broad waveband, since materials having sufficiently high thermal coefficients to be used in such coatings have lower indices of refraction generating lower interference effects. Optical elements with large numbers of layers tend to be extremely lossy due to the fact that diffraction and interference takes place at each individual layer interface.
One way to reduce the number of layers is to use materials with high indices of refraction to increase the ratio between the respective indices of refraction of the high index layers and the low index spacer material. However, while many materials having higher indices of refraction are available, such materials have low thermal coefficients of expansion, therefore making them susceptible to damage during the high temperature frit process as they are coupled to the laser tube. Thus, coatings utilizing a large high/low index ratio would have a tendency to craze due to the varying thermal gradients between the high index layers and the low index, or "spacer" layers and the laser mirror substrate material.
Another problem with conventional coatings stems from exposure damage from the internal environment of the completed resonant cavity. Ultraviolet radiation, generated by arc discharge within the laser tube, tends to deteriorate the coatings over time, reducing their effectiveness. In addition, the vacuum environment itself has a deleterious effect on the coating.
Thus, it is desireable to produce a laser resonant cavity through a method which avoids exposure of the finished optical elements to high temperature frit processing, and which has optical elements suitable for use over a wide bandwidth.